1. Technical Field
The present invention relates to devices and methods for use in orthopedic spine surgery. In particular, the present invention relates to a device having at least two distinct articulating surfaces, the device being useful as an artificial disc replacement and a method of implanting that device using a posterior approach.
2. Background Art
The human spine is comprised of thirty-three vertebrae at birth and twenty-four as a mature adult. Between each pair of vertebrae is an intervertebral disc, which maintains the space between adjacent vertebrae and acts as a cushion under compressive, bending and rotational loads and motions. A healthy intervertebral disc has a great deal of water in the nucleus pulposus—the center portion of the disc. The water content gives the nucleus a spongy quality and allows it to absorb spinal stress. Excessive pressure or injuries to the disc can cause injury to the annulus—the outer ring that holds the disc together. Generally, the annulus is the first portion of the disc that seems to be injured. These injuries are typically in the form of small tears. These tears heal by scar tissue. The scar tissue is not as strong as normal annulus tissue. Over time, as more scar tissue forms, the annulus becomes weaker. Eventually this can lead to damage of the nucleus pulposus. The nucleus begins to lose its water content due to the damage—it begins to dry up. Because of water loss, the discs lose some of their ability to act as a cushion. This can lead to even more stress on the annulus and still more tears as the cycle repeats itself. As the nucleus loses its water content it collapses, allowing the two vertebrae above and below to move closer to one another. This results in a narrowing of the disc space between the two vertebrae. As this shift occurs, the facet joints located at the back of the spine are forced to shift. This shift changes the way the facet joints work together and can cause problems in the facet joints as well.
When a disc or vertebrae is damaged due to disease or injury standard practice is to remove part or all of the intervertebral disc, insert a natural or artificial disc spacer and construct an artificial structure to hold the effected vertebrae in place to achieve a spinal fusion. In doing so, while the diseased or injured anatomy is addressed and the accompanying pain is significantly reduced, the patients mobility is also reduced, which may have an overall effect on their quality of life.
To that end, there is an overall need to treat the disease or injury while maintaining the six degrees of freedom inherent in the spine. Normal spine anatomy, specifically intervertebral disc anatomy, allows one vertebrae to rotate with respect to its adjacent vertebrae about all three axes. Similarly, the intervertebral disc also allows adjacent vertebrae to translate along all three axes, with respect to one another.
Few devices have been cleared for marketing as an artificial disc replacement by the Food and Drug Administration (FDA). The first device to be cleared by the FDA for use as an artificial disc replacement was the Charité® device by Depuy® Spine. This device has many shortcomings. First, the implantation method for the Charité® device requires an anterior approach to spine surgery. An anterior approach inherently increases the trauma to the patient because an incision must be made in the patient's abdomen and the internal organs must be pushed aside to allow access to the spine. Further, an anterior approach does not easily lend itself to revision surgery due to the amount of scar tissue present from the earlier surgery. This disadvantage is very likely to be encountered because the chance that a revision surgery will be required is higher than a normal fusion because one principal value of a motion preserving device, such as an artificial disc, is that is indicated for younger patients. Finally, Charité® device and surgical method is such that its initial implantation requires exact alignment due to necessary endplate preparation and if the implantation is performed incorrectly, a subsequent spinal fusion may be required.
Several other similar motion preserving devices have been recently cleared for marketing or are in various stages of clinical trials. These devices include the ProDisc® I and II devices by Synthes, Inc.®, the Maverick® and Prestige® devices by Medtronic Sofamor Danek®, the Flexicore® device by Spinecore, Inc.® and the Bryan Cervical Disc System® by Spinal Dynamics Corporation®. Similar to the Charité® device discussed above, each of these motion preserving devices requires an anterior approach to implant the device in a patient and thus, each of these devices share at least some of the common disadvantages to that procedure. Additionally, some of the devices discussed above utilize a metal-plastic or a plastic-plastic articulating surface wherein the plastic used is thin and typically a form of polyethylene. While articulating surfaces involving plastic have been very successful in other joint replacements, they are subject to varying degrees of implant life and particulate debris, which potentially can cause further damage to the body. Damage caused by particulate debris may become even more critical with spine implants due to the proximity of the spinal cord. Further, many, if not all, of the devices discussed above do not allow for the normal six-degrees of freedom inherent in a natural intervertebral disc.
For the above stated reasons, a need exists for an implantable articulating device, that can be introduced into the body using a posterior approach, similar to a PLIF, T-PLIF or X-PLIF spinal fusion device, can provide the normal six-degrees of freedom for the vertebrae adjacent to the implant, can provide a pro-longed life span in the body that can withstand early implantation, as is often indicated for younger patients, and will have a limited amount of particulate debris so as to reduce complications over the useful life of the device.